1. Field of the Invention
The present invention relates to a global positioning system (GPS) device and an ionosphere error estimation method thereof. More particularly, the GPS device and the ionosphere error estimation method thereof of the present invention accomplish estimation of a user ionosphere error by directly using ionosphere pierce point coordinates and ionosphere errors of reference base stations (BSs).
2. Descriptions of the Related Art
The global positioning system (GPS) mainly operates in the following principal: positioning signals are transmitted by satellites located in the outer space, and then a signal receiving device located on the earth can determine a position where the signal receiving device is currently located according to the positioning signals, thus accomplishing the positioning purpose. Because the GPS system can be widely used in various fields and is of great utility, related technologies are actively developed around the world. However, such a process of transmitting signals by satellites from the outer space to the earth tends to be interfered by various environmental factors, so errors may arise in the positioning result.
Specifically, the clock skew of the satellites themselves when the satellites transmit signals from the outer space to the earth, orbit deviations of the satellites, signal propagation through an ionosphere or a troposphere, and the clock error of the signal receiving device all have an influence on accuracy of the positioning result of the signal receiving device. Among these factors, errors caused due to propagation through an ionosphere have the greatest influence. Further speaking, signals propagating through the ionosphere will be seriously interfered by large amount of free electrons existing in the ionosphere. Even further, because free electrons distributed in the ionosphere are not in a regular or uniform way (actually in an irregular and time-varying way) and vary with the time and regions, which makes it difficult to estimate the interference on the satellite signals.
In the prior art, a primary solution to overcome the interference caused by the ionosphere is to dispose a dual-band receiver in the signal receiving device so that signals transmitted by the satellites in two different frequency bands can be received by the signal receiving device simultaneously and, accordingly, errors caused due to this environment factor can be eliminated. However, additional use of the dual-band receiver leads to a significantly increased cost, which hinders the widespread use of such signal receiving devices.
Additionally, there are also prior art solutions that use the Klobuchar model, the Satellite-Based Augmentation System, the Wide Area Differential Global Navigation Satellite System or the like to improve estimation and compensation of the ionosphere errors. However, the Klobuchar model can only eliminate about 50% of the ionosphere errors, so the accuracy it provides is still low; and because the approaches of using constant ionosphere grid points to calculate errors in the Satellite-Based Augmentation System and the Wide Area Differential Global Navigation Satellite System lack flexibility, they are not adaptive to abrupt variations of ionospheres in low latitude regions, which indirectly causes differences in terms of the using effect in different regions.
According to the above descriptions, an urgent need exists in the art to keep the signal receiving device at a low cost and, meanwhile, improve the accuracy of the ionosphere error estimation and the flexibility in use of the signal receiving device.